As known, hygienic products, in particular diapers for babies, sanitary towels or products for adult incontinence, comprise a layer of absorbent padding that is enclosed between a layer of nonwoven fibre and an impermeable layer, for example polyethylene. The absorbent padding is made of a conglomerate of cellulose fibres and/or of particles of superabsorbent material which is formed in an apparatus for forming such hygienic products.
In order to make anatomically shaped hygienic products, shaping the absorbent padding is known, according to the desired anatomical shape before enclosing the absorbent padding between the layer of nonwoven fibre and the impermeable layer.
The forming apparatus (which is not illustrated) comprises a forming drum 1 (illustrated schematically in FIGS. 1 to 3) of absorbent padding, the outer periphery of which is provided with a plurality of sucking forming pockets, and is supplied, at the periphery thereof, with a flow of particulate material. In each pocket, the fibres of the particulate material are conveyed by a flow of sucking air and are compacted by suction, thus obtaining the absorbent conglomerate, which is also known as fluff, of the required shape.
According to a different embodiment, which is not illustrated, the forming conveyor can comprise a closed loop continuous belt conveyor.
As shown in FIG. 1, the forming drum 1 comprises a plurality of forming pockets 2a shaped, aligned and uniformly distributed along the external surface of the drum 1, and comprising for example a cavity of substantially troncopyramidal shape, for making variable thicknesses of absorbent padding. Alternatively, as shown in FIG. 3, the forming drum 1 can comprise forming pockets 2b shaped, aligned and uniformly distributed along the external surface of the drum 1, and comprising for example an anatomical cavity of rounded shape for making absorbent padding of anatomical shape. Also alternatively, as shown in FIG. 2, the forming drum 1 can comprise a single forming pocket 2c shaped as a single annular cavity for making a web of absorbent padding, to be divided into rectangular portions with subsequent cutting.
Each forming pocket has in other words the desired shape for the padding to be obtained and/or to permit the successive processes for which the absorbent conglomerate is intended. The depth of the forming pocket determines the thickness of the absorbent layer to be made. The forming pockets are typically fixed to spaces of the forming drum of a shape corresponding to the pockets.
The forming pockets have to be perforated to enable the air flow to retain effectively by suction the particles of which the particulate material is made on the surface thereof, but at the same time they have to prevent the powdered material, which is also a component of the particulate material, from traversing the pockets. The openings in the forming pockets thus have to have a reduced dimension and typically it is required for the openings to have a dimension comprised in a range between 0.20 mm and 0.40 mm.
In order to make the forming pockets, which are usually made of metal, it is known to use, for receiving and retaining the particulate material, micro-perforated thin metal plates or micro-perforated metal nets by means of which it is possible to make openings of the desired dimension. Nevertheless, such thin metal plates or such metal nets have a reduced thickness and are thus flexible and easily deformable.
The deformability of the forming pocket during assembling and/or dismantling of the pocket in the forming drum makes handling and cleaning thereof difficult, which cleaning is frequently scheduled at set intervals in order to remove by means of a thorough cleaning any particulate material wedged in the openings of the forming pocket.
In order to ensure the appropriate toughness of the forming pockets, prevent possible deformation thereof and facilitate assembling and dismantling of the forming pockets during maintenance, it is known to support the thin micro-perforated metal plate or the metal net, which makes the forming substrate, by means of a stiff support, which is also perforated to permit the passage of air, supporting the external substrate.
The external substrate has a shape that is conjugated to the form of the absorbent padding to be made whereas the stiff support is of a shape conjugated to the external substrate shape to support appropriately the external substrate and confer toughness thereupon.
As shown by U.S. Pat. No. 4,761,258 and shown in FIGS. 4 and 5 with reference to a forming pocket 2a shaped as in FIG. 1, the external forming substrate 3 is made by a perforated thin metal plate, has a substantially frustoconical cavity 4 and is supported by a supporting structure 5, made as a metal net, which has a corresponding cavity 6, shaped like the cavity 4 and arranged at the cavity 4 of the external forming substrate 3. As an alternative to the metal net 5, the supporting structure can be made by a metal grid 7, shown in FIG. 5 or by means of a honeycomb grid (which is not illustrated), which provides greater toughness than the metal net and are thus usually preferred.
It should be noted that the absorbent padding that is obtainable from the forming pocket of FIG. 4 is a portion with a greater thickness at the frustoconical cavity 4 of the forming pocket 2a. Shaped padding of variable thickness forces the external forming substrate 3 to have at least one concave zone like the cavity 4 and consequently forces also the external surface of the supporting structure 5, intended to contact the external forming substrate 3 to have a respective concave zone, and i.e. the cavity 6. In addition, when the forming pocket is fixed to the forming drum, the inner surface of the supporting structure, opposite the external surface, is also curved because it is intended for contact with the forming drum 1.
The method for making the grid supporting structure thus has a plurality of production steps for making an external surface and/or an inner surface with hollow portions from a flat supporting grid structure, which make the grid supporting structure very expensive.
After obtaining a flat grid by welding together a plurality of drawn metal sheets, the grid is first curved to obtain the inner surface to be rested on the welding drum and is then treated by a process of electroerosion to make the hollow zones of the external surface.
This producing method thus makes it necessary to provide dedicated equipment. Special welding work benches are in fact necessary, on dedicated electroerosion machining work benches, which are very expensive. The electroerosion process is in itself very expensive because it requires dedicated electrodes that are to be regenerated frequently inasmuch as they are subject to wear.
In addition to the dedicated equipment, several machining steps are necessary, which make the production process of each grid supporting structure lengthy and which further require much labour.
It is added that the shape of each grid supporting structure is determined by the shape of the corresponding absorbent padding to be made and that accordingly the equipment dedicated to the production of a specific type of supporting structure has to be modified with the variation of the type of absorbent padding to be made.